I believe waterways that lead to reservoirs are technically illegal to fish in(unless stated) but authorities not too particular or keen to enforce the rules outside reservoirs. One canal I know you can legally fish is Pang Sua Canal located near woodlands road.

I went to Kallang River behind blk 63 recently and was surprised to see a huge Toman around 2.5 ft long swimming on the surface for 15 sec. There were also huge catfishes surfacing for breath. Later I walked upstream towards Bendemeer Road and saw an angler casting artificial lure appearantly aiming for Tioman. Unfortunately, there are presently "No Fishing" signs now at these spots. Maybe you could try explore further upstream to see whether there are areas upstream where yet to impose the no fishing sign. Good luck.